Medical instruments for minimally invasive procedures often employ mechanical wrists or joints that are remotely activated. In some configurations, a wrist mechanism may be part of or may manipulate a tool such as forceps, a scalpel, scissors, or a cauterizing tool that is at the distal end of a main tube of the instrument. Tendons generally attach to the mechanical members or disks in the wrist mechanism and extend through the main tube to a drive system at the proximal end of the main tube. U.S. Pat. No. 6,817,974, entitled “Surgical tool having Positively Positionable Tendon-Actuated Multi-Disk Wrist Joint,” to Cooper et al. describes some known wrist mechanisms containing multiple disks and tendon controlled wrist joints.
Geared movement in a wrist mechanism results at a joint when two members in the mechanism have relative angular orientations that change according to a fixed relationship or gear ratio. FIG. 1A, for example, shows a side view of a wrist joint 100 having members 110 and 120 with load-bearing surfaces 112 and 122 that are circular and have the same radius of curvature. Geared motion with a 1:1 gear ratio results when circular surfaces 112 and 122 roll on each other without slipping. To prevent slipping, a pin or tooth 114 on member 110 can engage the walls of an aperture 124 in member 120, and prevent members 110 and 120 from slipping relative to each other when a drive system (not shown) pulls on a tendon 130 and pays out an opposing tendon 132 as shown in FIG. 1B. Tooth 114 is preferably shaped, positioned, and sized so that tooth 114 stays coupled to aperture 124 over the full range of rotation of members 110 and 120. Geared motion is desirable because the movement of members 110 and 120 can be modeled and predicted in a computer or other processing system that determines how to manipulate tendons 130 and 132 to achieve a desired movement of joint 100. Circular load bearing surfaces 112 and 122 preserve the separation between the centers of circles defining surfaces 112 and 122, which is desirable because the resulting movement is easily modeled and because symmetric attachments of tendons 130 and 132 result in the length of tendon 130 or 132 pulled in being equal to the length of the other tendon 132 or 130 paid out. The mechanics of the drive system that manipulates tendons 130 and 132 can therefore be simplified.
Gears in general are known to use a variety of surface shapes that allow teeth of the gears to mesh without binding. For example, cycloid gears are gears having teeth with contacting surfaces following cycloidal curves. FIG. 2 shows a pair of cycloid gears 210 and 220 that are positioned on respective axles 212 and 222 that have the separation required for the teeth of gears 210 and 220 to correctly mesh. In particular, when gear 210 is driven to rotate clockwise, a surface 214 of gear 210 contacts a surface 224 of gear 220. Surfaces 214 and 224 are shaped to roll against each other as gears 221 and 220 rotate, e.g., surface 214 can be flat while surface 224 follows a cycloidal curve. Surfaces 216 and 226 then come into contact, and gears 210 and 220 rotate until the next tooth 218 of gear 210 comes into contact with the next tooth 228 of gear 220. Proper meshing of cycloid gears such as gears 210 and 220 is sensitive to the spacing of axles 212 and 222, which may be a leading reason why involute surfaces are much more commonly used in gears today.
Gears such as illustrated in FIG. 2 differ from a geared wrist joint such as illustrated in FIGS. 1A and 1B in that axles 212 and 222 fix the spacing of gears 210 and 220, so that contacting surfaces of the gears do not bear significant compression forces. In contrast, wrist joint 100 of FIGS. 1A and 1B when in a medical instrument would often need to bear compression forces (e.g., due to tensioning of both tendons 130 and 132) that tend to flatten the convex load bearing surfaces 112 and 122 of members 110 and 120. Since medical instruments containing wrist joints such as wrist joint 100 often have relatively small diameters, e.g., typically on the order of 3 to 10 mm for the entire diameter of member 110 or 120, and may encounter compression forces on the order of one hundred pounds, deformations of load bearing surfaces 112 and 122 are important concerns. Further, much of the cross-sectional area of a wrist joint may be needed for tendon guides or for one or more central lumens for devices such as camera cables or other systems that may extend through the instrument. The small available space for mechanical structures can make it challenging to manufacture wrist joint 100 with suitable strength and precision. In particular, the relatively small size of tooth 114 can result in tooth 114 disengaging from aperture 124 at the extremes of wrist motion, particularly when wrist 100 supports a load that deforms contacting surfaces 112 and 122.